Micro Structural Investigations and Mechanical Properties of Macro Porous Ceramic Materials from Capillary Suspensions

Recently, we have introduced a novel, material‐independent processing method for producing macro porous ceramics with capillary suspensions as a stable precursor. A capillary suspension is a three‐phase system where a small amount of an immiscible secondary liquid is added to a suspension resulting in the formation of a sample spanning particle network. This technology provides open porosities well above 50% and pore sizes ranging from 0.5–100 μm. Here we focus on microstructure formation in the capillary suspensions and its impact on mechanical strength of the corresponding sintered parts. Based on the rheological data and SEM‐images, three regimes (I, II, III) are identified with distinctly different flow properties of the wet suspension and characteristic structural features of the sintered ceramic parts depending on the amount of added secondary liquid phase. The average pore size increases and the pore size distribution changes from monomodal (I) to bimodal (II) and broad multimodal (III) with increasing amount of secondary liquid phase. A clear correlation between the yield stress of the wet suspension and the porosity and pore size is observed for regime (I) and (II). Compressive and flexural strength as well as the Young's modulus monotonically decrease with increasing amount of the secondary liquid phase. Absolute values are mainly determined by the porosity and are well predicted by the Gibson & Ashby model for samples corresponding to regime (I) and (II). The broad pore size distribution in regime (III) results in a significantly lower mechanical strength.     

Thermoreversible Gelcasting: A Novel Ceramic Processing Technique

A new ceramic processing method, thermoreversible gelcasting (TRG), has been developed. The method uses a gelation process that can be reversed multiple times. Two Al₂O₃ systems have been explored with the use of a specific triblock copolymer to form a binding gel network. The mixture becomes a free-flowing liquid on heating above 60°C. However, on cooling below 60°C, the slurry instantaneously transforms to a physical gel. This process can be reversed easily (essentially with an infinite processing time window) while the solvent is present in the system, which is advantageous when trying to produce high-quality dense pieces if initial casting irregularities occur. Near-theoretically dense specimens have been produced with properties consistent with reported values of high-density Al₂O₃.     

Mullite Suspensions for Reticulate Ceramic Preparation

The dispersing behavior of MgO-doped mullite suspension suitable for the production of reticulate ceramics by sponge impregnation was studied. Three deflocculants were tested and the optimum dispersing conditions for the mullite suspension were found. Electrokinetic studies of magnesia in water evidence that it undergoes pronounced solubilization that strongly affects the performance of the mullite suspension when added at the 1 wt% concentration, in terms of solid loading and aging effects. The optimum composition of the slurry was found, reticulate mullite samples were produced, and the green and sintered bodies were characterized.     

制粉设备和工艺流程对产品质量的影响

原料的质量是产品优质、高产的前提和基础。本文从原料、储浆、球磨和喷雾塔等各个工序的控制入手,分析了各种影响因素并提出解决的办法,以获得稳定、优质的原料,最终实现提高产品质量的目标。     

Design of Ceramic Materials for Chemical Sensors with Novel Properties

Recently, the search for novel functions has received increasing attention in the field of materials. Outstanding possibilities for innovative applications are given by electroceramics. One of the fields which may be improved by a different use of functional ceramics is that of chemical sensors. In fact, the materials conventionally used in this field show problems still to be solved, such as insufficient gas selectivity, inability to detect very low gas concentrations, and changes in sensing properties caused by surface contamination. One possible solution to these problems is the design of materials with novel detection mechanisms. The design of innovative properties may be achieved by integration or hybridization of materials with different properties. In this paper, examples of multiphase materials for humidity and gas sensors are reported, i.e., La₂CuO₄/ZnO p-n heterocontacts, Au/ZnO Schottky barriers, and sol-gel processed thin films of TiO₂ with 10 at. % of K, based, respectively, on the interaction of p -type/ n -type semiconductors, metal/ceramic, and conductor/insulator.     

A Novel Method Based on Capillary Suction Time (CST) for Assessment of Dispersion Characteristics of Suspensions

This paper presents the applicability of a simple and novel technique based on capillary suction time (CST) to characterize the dispersibility of suspension. The method is simple, rapid, and requires little experience. It has been shown to be extremely useful for comparing the dispersibility during preparation of a stable suspension of ceramic powder with high solids loading. The method has also been found to be suitable for obtaining homogeneous suspensions of maximum stability. The dispersibility of particles in any suspension has been assessed with respect to the condition at point of zero charge (pH_pzc). A quantitative measure of the dispersibility has been proposed in term of a dispersion ratio (DR). According to this proposition, the suspension is in a state of dispersion when the value of DR is more than unity, whereas it is in a flocculated state when the value of DR is less than unity.     

Process Mechanism for Vacuum-Assisted Microfluidic Lithography with Ceramic Colloidal Suspensions

The infiltration kinetics and the drying mechanism of suspensions involved in vacuum-assisted microfluidic lithography (μFL) have been investigated for the fabrication of complex micropatterned ceramic structures. Infiltration lengths of alumina suspensions with various solid loadings into microchannels were analyzed as a function of channel widths ranging from 10 to 100 μm. The use of well-dispersed ethanol-based suspensions with lower viscosities and wider channels allowed for easier and longer infiltration due to lower fluidic resistance in the channels. In contrast to the micromolding in capillaries (MIMIC) method, vacuum-assisted μFL has a distinct drying mechanism in which there is a critical level of solid loading of the suspension with respect to the volume of the microchannel for the fabrication of a defect-free pattern structure.     

Cold Sintering Process: A Novel Technique for Low‐Temperature Ceramic Processing of Ferroelectrics

Research on sintering of dense ceramic materials has been very active in the past decades and still keeps gaining in popularity. Although a number of new techniques have been developed, the sintering process is still performed at high temperatures. Very recently we established a novel protocol, the “Cold Sintering Process (CSP)”, to achieve dense ceramic solids at extraordinarily low temperatures of <300°C. A wide variety of chemistries and composites were successfully densified using this technique. In this article, a comprehensive CSP tutorial will be delivered by employing three classic ferroelectric materials (KH₂PO₄, NaNO₂, and BaTiO₃) as examples. Together with detailed experimental demonstrations, fundamental mechanisms, as well as the underlying physics from a thermodynamics perspective, are collaboratively outlined. Such an impactful technique opens up a new way for cost‐effective and energy‐saving ceramic processing. We hope that this article will provide a promising route to guide future studies on ultralow temperature ceramic sintering or ceramic materials related integration.     

Novel Processing of 1-3 Piezoelectric Ceramic/Polymer Composites for Transducer Applications

Piezoelectric ceramic/polymer composites with 1-3 connectivity were made by weaving sized lead zirconate titanate (PZT) fiber bundles through a honeycomb support. Bundles comprised of fine-scale, 20-50 μm green fibers, made using the viscous suspension spinning process, were sized to increase their manageability. The sizing step comprised of soaking the green PZT fiber bundles in an aqueous solution of poly(vinyl alcohol), then pulling the wet fibers through a steel sizing die. Sizing resulted in dense and flexible fiber bundles, which facilitated composite construction and led to composites with increased volume fractons. Sintering, polymer embedding, and machining produced a composite exhibiting 1-3 connectivity. Composites with 10 vol% PZT yilded d ₃₃ values of 230 pC/N and a dielectric constant of 130.     

Processing Routes to Macroporous Ceramics: A Review

Macroporous ceramics with pore sizes from 400 nm to 4 mm and porosity within the range 20%–97% have been produced for a number of well-established and emerging applications, such as molten metal filtration, catalysis, refractory insulation, and hot gas filtration. These applications take advantage of the unique properties achieved through the incorporation of macropores into solid ceramics. In this article, we review the main processing routes that can be used for the fabrication of macroporous ceramics with tailored microstructure and chemical composition. Emphasis is given to versatile and simple approaches that allow one to control the microstructural features that ultimately determine the properties of the macroporous material. Replica, sacrificial template, and direct foaming techniques are described and compared in terms of microstructures and mechanical properties that can be achieved. Finally, directions to future investigations on the processing of macroporous ceramics are proposed.     

Ceramic Membranes for Novel Separations

Abstract

A number of advances have been made in the fabrication and performance of supported ceramic membranes. Improvements in techniques for the synthesis of particulate sols containing particles in the range of 4 to 10 nm in diameter are discussed. Several deposition techniques for the fabrication of supported ceramic membranes via solgel technology are examined, including slipcasting, filtration, coating, and permformation.

Supported titania membranes have been fabricated and both liquid phase permselectivity and gas phase permeability experiments have been performed. Results of the liquid phase separations indicate that these membranes are characterized by a molecular weight cutoff value of approximately 200. Gas phase permeabilities reveal that Knudsen diffusion is not the only mechanism of transport through these membranes.     

陶瓷膜过滤细微颗粒悬浮体系的数学模拟

通过对陶瓷膜过滤悬浮液过程中单颗粒的受力分析,获得了可沉积颗粒临界粒径(xcrit)的计算公式.计算的xcrit值与实验测定的沉积层内的粒径分布一致:随颗粒粒径的增大, xcrit呈先增大后减小的趋势. 在讨论xcrit与颗粒粒径(dp)、膜孔径(dm)关系的基础上,将微滤过程按dp/dm的比值分为三种类型:dp/dm≤1, 颗粒迅速阻塞膜孔;1＜dp/dm≤10 ,颗粒在膜表面形成连续滤饼;dp/dm》10,颗粒对膜孔口覆盖,在膜表面形成不连续滤饼.并以此为基础建立了预测陶瓷膜过滤不同粒径和粒径分布的颗粒悬浮液渗透通量的数学模型.计算结果与实验结果的比较表明:本模型对文献模型的改进之处是适用范围从1＜dp/dm≤10扩展到0.48≤dp/dm≤120;在操作压力对渗透通量的影响计算中,假设滤饼层孔隙率不变的计算结果与实验结果一致,说明此体系的滤饼是不可压缩的;在错流速率对渗透通量的影响计算中,假定滤饼层孔隙率随错流速率成指数变化的计算结果比假定沉积层孔隙率随错流速率不变的计算结果好,说明错流速率的变化同时影响滤饼层内颗粒的粒径分布和其形成的滤饼层的结构.     

A Novel Processing Route to Control Grain Growth in Submicrometer Alumina Compacts

A novel processing procedure for significantly suppressing grain growth in submicrometer alumina compacts has been developed and implemented with the intent of ultimately using the same processing route to control grain size in nanophase alumina compacts. In this study, partially sintered alumina pellets made from 0.5 µm starting powders are altered by the chemical infiltration of Si₃N₄. The control and infiltrated pellets are then heated to 1650°C for 4 h. The fully sintered pellets are approximately 97% dense. Suppressed grain growth is observed in the infiltrated pellets. The average grain size in the control pellets after densification is 4.2 and 1.2 µm in the infiltrated pellets. Depth of infiltration is measured using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Depending on the specific infiltration conditions used, the outer 15-50% of the infiltrated pellets exhibit a graded microstructure consisting of a region of abnormal grain growth and a region of suppressed grain growth. Abnormal grain growth is visible on the outer surfaces of the infiltrated pellets where a relatively high ratio of Si to N is present. Further into the pellet, after some depletion of the Si source gas has occurred, regions of suppressed grain growth are apparent. Based on these results, an infiltration profile is determined. A mathematical model is developed to describe the infiltration process and to determine optimal infiltration conditions. High-resolution electron microscopy (HREM), energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS) are used to study the infiltrated samples.     

Specific Volume Diagrams for Ceramic Processing

Graphical procedures are presented for analyzing the volume-weight relations in ceramic particulate systems encountered in processing. The procedures emphasize the advantages of working with specific volume rather than density. By using the concept of partial specific volumes, the expected variations in specific volume with composition for many special cases are readily deduced. Comparing experimental and theoretical variations gives meaningful insights into the space-filling characteristics of real systems.     

Method for Processing Metal-Reinforced Ceramic Composites

A new process is developed to form a ceramic containing a three-dimensional network of metal reinforcement. The process involves four steps: (1) forming a powder compact containing a continuous network of either organic or carbon material by pressure filtration, (2) pyrolyzing the network to form channels within the powder compact, (3) densifying the powder while retaining the channel network, and (4) intruding metal into the channel network by squeeze casting. Pressure filtration is used to form the powder compact containing the pyrolyzable network either by mixing slurries of powder with chopped fiber or by packing powder within pyrolyzable preforms. When pressure is removed after filtration, the differential strain recovery of the powder matrix relative to the organic material can cause damage. Such damage is prevalent for a powder matrix formed from flocced slurries. However, this problem is avoided by using dispersed slurries which produce consolidated bodies that alleviate stresses arising from differential strain recovery by viscous flow. Metal-reinforced ceramic composites with different reinforcement architectures, volume fractions, and sizes can be produced with this technique.     

Novel Structural Modulation in Ceramic Sensors Via Redox Processing in Gas Buffers

High selectivity, enhanced sensitivity, short response time, and long shelf-life are some of the key features sought in the solid-state ceramic-based chemical sensors. As the sensing mechanism and catalytic activity are predominantly surface-dominated, benign surface features in terms of small grain size, large surface area, high aspect ratio and, open and connected porosity, are required to realize a successful material. In order to incorporate these morphological features, a technique based on rigorous thermodynamic consideration of the metal/metal oxide coexistence is described. By modulating the oxygen partial pressure across the equilibrium M/MO proximity line, formation and growth of new oxide surface on an atomic/submolecular level under conditions of "oxygen deprivation," with exotic morphological features, has been achieved in potential sensor materials. This paper describes the methodology and discusses the results obtained in the case of potential semiconducting ceramic oxide-based carbon monoxide and hydrogen sensors with enhanced characteristics.     

Viscosity and Acoustic Behavior of Ceramic Suspensions Optimized for Phase-Change Ink-Jet Printing

The application of ink-jet printing technologies to direct-write and freeform fabrication of ceramics requires the development of appropriate printing fluids. The critical parameter controlling fluid printing ability is viscosity, although surface tension and density are also important. Additionally, in piezoelectric-driven ink-jets, drop ejection is governed by a series of electric pulses exciting a transducer at acoustic frequencies and hence the fluid acoustic wave speed is also important. Here, suspensions of sterically stabilized fine ceramic powders in solidifying hydrocarbon systems exhibiting suitable rheology for ink-jet printing have been formulated and their acoustic behavior has been characterized. For the particle sizes and frequencies used in this study, acoustic wave speed as a function of solids content is accurately predicted by effective medium theory. Suspensions containing up to 45% particulate by volume can be successfully passed through conventional printing heads.     

Thermal-Stress Crack-Initiation Method for Ranking Ceramic Materials

A test is described which permits ranking of brittle materials according to their resistance to thermal fracture under steady-state conditions. A water-cooled probe and a small, thin, annular disk specimen heated by a radiator are used. Tests on AUC graphite and on a series of composites of transition-metal pseudobinary solid-solution carbides with graphite were very reproducible and reliable. A thermal-stress analysis for the graphite agreed well with the test results.